Optical electric field or voltage sensing system

ABSTRACT

An electric field sensing system is characterized so that a signal processor may derive an output representative of an electric field intended to be measured, particularly established by high voltage on a power line, with enhanced accuracy. Characterizing parameters may be derived from signals representative of a pair of light waves derived from a light wave exiting an optical electric field sensing element in a electric field sensing head spatially positioned in the high voltage environment.

RELATED APPLICATION

This application claims the benefit of priority pursuant to 35 USC 119of provisional patent application Ser. No. 60/337,820 filed Nov. 13,2001, the disclosure of which application is hereby incorporated in itsentirety by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to optical sensors, and moreparticularly to those optical sensors that are responsive to an electricfield or voltage, and systems incorporating such sensors for reliableand very accurate measurement of electric field and/or voltage.

DESCRIPTION OF THE INVENTION

Optical technology has attracted a great deal of attention in the pastfew decades due to various attractive features that it offers. Thesefeatures may present great advantages over other technologies used forthe same applications depending on the requirements of that application.For example, optical fibers are now widely used in the telecommunicationindustry due to various benefits that they offer including their lowloss compared to electrical cables (copper wire), which allows for longcable runs before needing to amplify or regenerate the signals (muchfewer repeaters needed). Other examples are optical sensors used inharsh environmental conditions of chemical or nuclear industry. Inertglass optical fiber is a great means of transporting signal (and data)without being affected by the chemical reaction being monitored, forexample.

More specifically, in the electric power industry, optical fibersprovide a great deal of attractive benefits, which may include thefollowing: immunity to electromagnetic interference, galvanic isolationof sensors at high-voltage (HV) from ground potential (safety for theworkers and equipment), galvanic isolation between HV lines (safety andreliability for equipment).

Optical voltage and current sensors in the HV industry may also offerthe following advantages, among others, over conventional voltage andcurrent transducers: lighter weight—resulting in faster and easierinstallation, and better seismic withstanding, higher accuracy, largerdynamic range of measurement, wider bandwidth, safer insulation designdue the wide separation of HV and ground, use of more environmentallyfriendly insulation—elimination of need for oil and/or SF₆ gas forinsulation.

Prior art optical voltage sensors and systems commonly employ opticalelectric field sensors to measure electric field, and, of course,voltage—the potential difference or voltage establishing the electricfield. Prior art optical voltage sensors and systems, for example, aretaught, among many others, in U.S. Pat. No. 6,252,388, entitled, “Methodand Apparatus for Measuring Voltage Using Electric Field Sensors, andU.S. Pat. No. 6,380,725, entitled, “Voltage Sensor, both of which areincorporated herein by reference in their entirety. Commonly, such priorart optical voltage sensing systems include a sensing head and a remotesystem module connected via optical fibers for transmitting andreceiving optical signals to and from the sensing head. It should beappreciated that the optical high voltage sensing head is generallyintended to be spatially positioned within an insulator column, althoughnot shown or described, herein, so as to be responsive to an electricfield established by the voltage difference across the column as is wellunderstood in the art. The remote system module, including lightsources, optical signal detectors, signal processors, and the like, isgenerally spatially positioned away from the harsh high voltageenvironment.

As taught in the aforementioned patents, a voltage sensor employs anoptical electric field sensing element that exhibits an electro-opticeffect where transit time of light wave components of differingpolarizations are affected differently in response to an electric field.One form of an optical electric field sensor, herein referred to as anEFS, is a Pockels Cell type sensor, similar to that disclosed in U.S.Pat. No. 5,029,273, entitled, “Integrated Optics Pockels Cell VoltageSensor,” the disclosure of which is incorporated by reference. Ofcourse, other types of Pockels Cell type optical electric field sensingelements may be employed including, among others, bulk-optic sensorscomprised of a crystalline material where differing polarization lightwave components are affected differently in response to the presence ofan electric field.

As indicated earlier, optical electric field sensing elements employedas part of an optical high voltage sensing head are particularlyadvantageous in the extreme high voltage environment. This is so, sinceonly optical components are spatially positioned in the high voltageenvironment, including among others, optical fibers to route light wavesinto and out of the optical sensing element. In turn, opticalphotodetectors, signal conditioning circuitry, and signal processors toderive an output indicative of the voltage or electric field intended tobe measured, are spatially positioned in the more benign electric fieldenvironment away from high voltage anomalies.

Although many improvements have been made in optical high voltagesensors and sensing systems, there remains a need for improved accuracyEFS sensing systems that have diminished sensitivities to lifetime andtemperature variations in both electrical and optical system components.For example, optical component variations include those induced byambient temperature variations, for example, variation in the power ofthe light source that produces the input light wave to the sensing head;and electrical component variations, for example signal conditioningamplifiers and photodetectors, before digital signal processing.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a method and apparatus forenhancing optical voltage sensing system accuracy for lifetime andtemperature induced changes in optical and electrical components of thesensing system.

In accordance with the present invention, an electric field sensingsystem for measuring an electric field includes a optical sensing headincluding (i) an optical electric field sensing element or sensor thatexhibits an electro-optic effect where transit time of light waves ofdiffering polarizations are affected differently in response to anelectric field, (ii) an input optical circuit responsive to an inputlight wave for coupling first and second light wave components to theoptical sensing element, and (iii) an output optical circuit coupled tothe optical sensing element for splitting light exiting therefrom intothird and fourth output light waves derived from the first and secondlight wave components, and directing said third and fourth light wavesto impinge upon a first and second photodetectors, respectively. Asignal processor derives a processor output signal, E, representative ofthe electric field intended to be measured as a sensing-system-specificcharacterized function, G_(x), mathematically described by:E=G _(x) {a, b, E _(π), φ_(o) , K,}where,

-   -   and b represent resultant signal processor input signals derived        from the pair of photodetectors of the electric field sensing        system, including both optical and electrical signal losses        along the way from the output of the optical electric field        sensing element,    -   E_(π) is a sensing system specific parameter being substantially        the average electric field required to a create π radians phase        shift between first and second light wave components traveling        from the input to the output of the optical sensing element in        response to an electric field,    -   φ_(o) is the intrinsic or inherent phase shift between the first        and second light wave components derived from the signals a and        b, and    -   K is equal to the ratio K_(A)/K_(B), where K_(A) and K_(B) are        characteristic loss factors of the specific electric field        sensing system, X, associated with said first and second        processor input signals, respectively, derived from the signals        a and b.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary embodiment of an optical electric field sensingsystem in accordance with the present invention.

FIG. 2 is a graphical depiction illustrating the relationship between aninput electric field, E, intended to be sensed, and resulting outputsignals, a and b, for a sinusoidal 60 Hz signal electric field orvoltage signal intended to be sensed.

FIG. 3 is a graphical depiction illustrating the transfer function ofequation 4.

FIG. 4 is a graphical depiction illustrating the relationship of theresidual bias, Δφ_(o), as a function of temperature for an opticalelectric field sensor.

FIG. 5 is a graphical depiction illustrating a plot of R described byequation 12 as a function of temperature.

FIG. 6 is a graphical depiction illustrating a plot of R as a functionof Δφ_(o), combining the data from FIGS. 4 and 5, with a straight linerepresenting a linear fit to the data.

FIG. 7 is a graphical depiction illustrating a plot of the inverse of R,namely 1/R, as a function of Δφ_(o), with a straight line representing alinear fit to the data.

FIG. 8 is a graphical depiction illustrating error in the measurement ofan AC electric field intended to be measured as a function of the trueΔφ_(o) residual bias.

FIG. 9 is a block diagram of an exemplary signal processor in accordancewith the present invention.

FIG. 10 is a block diagram of an exemplary digital signal processor ofthat depicted in FIG. 9.

FIG. 11 is another block diagram of an exemplary digital signalprocessor of that depicted in FIG. 9 having further optional functionsin accordance with the present invention.

FIG. 12 is another block diagram of an exemplary digital signalprocessor of that depicted in FIG. 9 arranged for providing a zerowake-up time function.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is a block diagram of one embodiment of an opticalelectric field sensing system 100 in accordance with the presentinvention including (i) an optical electric field sensing head indicatedby block 10, in dashed line form, and (ii) a remote system modulegenerally indicated by block 50, in dashed line form, including opticalsignal source 55, a pair of optical signal photodetectors, 62, 64, and asystem signal processor 800.

The optical sensing head 10 is generally spatially positioned in a highvoltage environment intended to measure an electric field established bya high voltage power line (not shown). The remote system moduleindicated by block 50 is generally spatially located in a “lower”voltage environment away from or shielded from the high voltageanomalies associated with power lines.

The optical sensing head 10 includes an optical electric field sensingelement 12 illustrated as an electro-optic crystal where the speed oflight for certain polarizations differs upon application of an electricfield—e.g., an electro-optic crystal. Optical sensing head 10 furtherincludes an input optical circuit for deriving from an input opticalwave, I₀, from source 55 through optical fiber 57, a pair of first andsecond light wave components 32 and 34, and coupling these components toan input end of the optical sensing element 12; and an output opticalcircuit for splitting light exiting sensing element 12 into third andfourth light waves 47, 49, and directing these light waves to travelalong optical fibers 58 and 59, respectively, so as to impinge uponphotodetectors 62 and 64 respectively.

The exemplary input optical circuit illustrated in FIG. 1 includescollimating lens 22 for collimating the light out of the input fiber 57,polarizer 24 to establish a pair of linearly polarized light wavecomponents 25, 27, and a phase retarder or phase separator in the formof a quarter-wave plate 26 to convert the pair of linearly polarizedlight waves to a circular polarized light wave having first and secondlight wave components 32 and 34. It should be noted that phase adjuster26 establishes an intrinsic phase difference between the light wavecomponents, along with any intrinsic phase difference induced by thesensing medium, as the light wave components travel through the sensingmedium.

The exemplary output optical circuit illustrated in FIG. 1 includes apolarizing beam-splitter 42 to establish third and fourth light wavecomponents 47 and 49, which in turn are coupled to respective lenses 43and 45, respectively, for directing light into optical fibers 58 and 59,respectively, so that light therefrom impinges upon photodetectors 62and 64, respectively.

As should be well understood by those skilled in the art, it ispreferable that the total intrinsic phase difference between the firstand second light wave components from input to output is 90 degrees, andmay be implemented by imparting a 90 degree phase shift along theoptical path from the first optical circuit to the second opticalcircuit before the third and fourth light wave components are derived.This phase shift may be imparted on the first and second light wavecomponents either before or after, or throughout the sensing medium, ordistributed both before and after the sensing medium.

It should be noted that the input and output optical circuit of thesensing head maybe constructed by way of a variety of components beyondthat just described so as to achieved the intended function as alreadydescribed, all of which is intended to be within the true spirit andscope of the preset invention.

Light source 55, the detection unit including photodetectors 62 and 64,and the signal processor 800 are usually constructed of electronicand/or opto-electronic components, and may be integrated in oneelectronics module or circuit card as is well known to those skilled inthe art. Optical fibers 57, 58, and 59 effectively guide light wavesbetween block 50 and the sensing-head 10 in the high-voltageenvironment. Use of optical fibers provides good isolation between thehigh voltage environment of the optical electric field sensing head 10and the remote environment where the remote system module 50 is intendedto reside.

It should be recognized that FIG. 1 illustrates the exemplary electricfield sensing application where the sensor-head 10 is in the harshenvironment of a high-voltage environment. The present invention isapplicable to any harsh environment where isolation between thesensing-head and the remote system module 50 is desired, e.g., achemically active or radioactive or any other unsafe environment, wherethe sensor-head may survive but the remote system module would not.

FIG. 1 illustrates the particular case where the optical electric fieldsensing element or sensor, herein referred to as the EFS, is a Pockelscell type optical electric field/voltage sensing element. The outputoptical circuit derives from the output light wave components 36 and 38exiting from the electric field sensing element 12 two complementaryoptical output signals L_(A) and L_(B), light wave components 47 and 49,respectively. The first optical output signal or light wave, L_(A), isthe component of the sensing element 12 output that is polarized so asto be transmitted through the polarizing beam-splitter 42—polarizedparallel to one axis of the polarization ellipse of the output of theelectro-optic crystal as illustrated in FIG. 1. The second opticaloutput signal or light wave, L_(B), is the component of the sensingelement 12 output that is polarized so as to be reflected by thepolarizing beam-splitter—polarized parallel to the other axis of thepolarization ellipse of the output light. Each of these light waves areoptically transmitted via optical fibers 58 and 59 from the sensor-head10 to remote system module 50, and specifically photodetectors 62 and 64for converting optical signals to electrical signals, “P_(a)”, and“P_(b),” for subsequent signal processing as will be further described.

Details of signal processor 800 are more particularly illustrated in theblock diagram of FIG. 9. The output electrical signals P_(a), and P_(b)from photodetectors 62 and 64, respectively, are typically signalprocessed by signal conditioning circuitry 910 a and 910 b,respectively, that may include amplifiers, limiters, filters, and thelike, for obtaining a useful signal for digitalization. In turn eachoutput of the signal conditioning circuitry 910 a and 910 b is presentedto a respective analog-to-digital converter 912 a and 912 b,respectively, having digital outputs D_(a) and D_(b), respectively.

As illustrated in FIG. 9, signal processor 800 further includes adigital signal processor 920 for processing the digital signals D_(a)and D_(b). Digital signal processor 920 may be constructed by way of awide variety of digital signal processors including microprocessors,computer, firmware, software, and the like for achieving the intendedsignal processing as will now be described.

In the following exposition, at any sampling time, signal “a” is adigital value representing light intensity L_(A) after having beeneffected by all the losses and scale factor changes through itstransmission and conversion from optical intensity to an electricalanalog or digital signal value; i.e., a=K_(A)×L_(A). Similarly,b=K_(B)×L_(B). In accordance with the present invention, signalrepresentations “a” and “b” are the resultant signals or measurementsrepresented by D_(a) and D_(b), respectively, operated on by digitalsignal processor 920 in order to determine the electric field or voltageintended to be measured.

A transfer function of the optical electric field sensing systememploying a Pockels cell EFS, hereafter referred to as the EFS sensingsystem, may be mathematically described by: $\begin{matrix}{a = {\frac{P_{0}K_{A}}{2}\left\lbrack {1 + {\alpha\quad{\cos\left( {\frac{\pi\quad E}{E_{\pi}} + \varphi_{0}} \right)}}} \right\rbrack}} & \text{(1.a)} \\{b = {\frac{P_{0}K_{B}}{2}\left\lbrack {1 - {\alpha\quad{\cos\left( {\frac{\pi\quad E}{E_{\pi}} + \varphi_{0}} \right)}}} \right\rbrack}} & \text{(1.b)}\end{matrix}$where,

-   -   a and b represent resultant signals derived from the        photodetector output signals (D_(a) and D_(b))of the electric        field sensing system, including both optical and electrical        signal losses along the way from the output of the optical        electric field sensing element,    -   P_(o) is the total optical power intensity of the input wave I₀        entering the sensor-head,    -   E is the electric field along the length of the electro-optic        crystal or the average electric field over the chosen sensor        length,    -   E_(π) is the (average) electric field required to create π        radians phase shift between orthogonal optical polarization        components traveling in the electro-optic crystal (32, 34),    -   φ_(o) is the bias or intrinsic phase shift between the optical        polarization components traveling in the electro-optic crystal        (mostly controlled by the quarter-wave plate in the embodiment        shown in FIG. 1),    -   α is a number between 0 and 1 (very close to 1 for a good EFS),        and    -   K_(A) and K_(B) are the loss factors for signals a and b,        respectively.

E_(π) is basically a measure of the sensitivity of the EFS and isdependent on many physical and geometrical parameters such as opticalwavelength, the electro-optic coefficients of the electro-optic crystal,its refractive index, and many other parameters. For a comprehensivediscussion on electro-optic phase modulation and sensors, reference ishereby made to T. Tamir (Ed.), entitled, “Guided-Wave Optoelectronics”,2^(nd) Edition, Springer-Verlag, 1990, and A. Yariv, and P. Yeh,entitled, “Optical Waves in Crystals”, John Wiley & Sons, 1984, whichare herein incorporated by reference in their entirety.

The value of α, a measure of the goodness of the EFS within the sensorhead, is usually less than 1.0, the value for an ideal EFS. However, thevalue of α is more typically slightly less than 1.0, e.g., 0.99, due topractical limitations in manufacturing an EFS, e.g., misalignment of theoptical components in the sensor-head.

The value of φ_(o), a measure of the bias or initial phase shift betweenthe two optical polarization components traveling in the electro-opticcrystal, is mostly determined by the quarter-wave plate which istypically designed so that φ_(o) is close to π/2 radians or oddmultiples thereof. In these circumstances, the optical electric fieldsensing system transfer function of equations 1.a and 1.b may berewritten as follows: $\begin{matrix}{a = {\frac{P_{0}K_{A}}{2}\left\lbrack {1 + {\alpha\quad{\sin\left( {\frac{\pi\quad E}{E_{\pi}} + {\Delta\quad\varphi_{0}}} \right)}}} \right\rbrack}} & \text{(2.a)} \\{b = {\frac{P_{0}K_{B}}{2}\left\lbrack {1 - {\alpha\quad{\sin\left( {\frac{\pi\quad E}{E_{\pi}} + {\Delta\quad\varphi_{0}}} \right)}}} \right\rbrack}} & \text{(2.b)}\end{matrix}$where,

-   -   Δφ_(o)=φ_(o)−π/2, the residual bias of the EFS.

Commonly, a Pockels cell EFS system uses only one of the two outputcomponents, a or b, as illustrated in FIG. 1, and mathematicallydescribed by the sensing system transfer function equation pairs 1 and2. FIG. 2 is a graphical depiction illustrating the relationship betweenan input electric field, E, intended to be sensed, and resulting outputsignals, a, and b, for a sinusoidal 60 Hz signal:E=E _(o) sin(ωt)  (3)Where,

-   -   E_(o) is the amplitude of the input electric field,    -   ω is the angular frequency, where ω=2πf,    -   t is time, and    -   f is the frequency of the electric field, here 60 Hz.

The purpose of signal processor 800 is to accurately determine a value Erepresentative of the electric field intended to be measured usingresultant signals a and/or b, in accordance with some form of apredetermined sensor transfer function. It is important to note thatsignals a, b, and various parameters of the transfer function of the EFSsystem may change due to changes in various parameters such astemperature, change of intensity of the input light source, P₀, andvibration, and the like. In accordance with the present invention, theoptical electric field sensing system and its associated signalprocessor 800 provide an accurate sensor electric field measurement “E”by taking into consideration the aforesaid effect of changing parametersthat may affect the overall transfer function of the EFS sensing system.

One parameter, the input light intensity, P₀, may vary or fluctuate dueto vibration, aging of the light source, or other causes. In accordancewith the present invention, the effect of variation in the input lightintensity, P₀, on the resultant measurement of E may be diminished by“normalizing” the EFS sensing system output signals, a and/or b, so thatresultant sensor system output value E may be derived such that it willnot be sensitive to optical light intensity or power fluctuations. Usingequations (2.a) and (2.b), a “normalized transfer function” may bederived by dividing the EFS sensing system signal outputs a by (a+Kb),where K=K_(A)/K_(B) as follows: $\begin{matrix}{S_{A} = {\frac{a}{a + {Kb}} = {\frac{1}{2}\left\lbrack {1 + {{\alpha sin}\left( {\frac{\pi\quad E}{E_{\pi}} + {\Delta\quad\varphi_{0}}} \right)}} \right\rbrack}}} & (4)\end{matrix}$

It should be noted that other forms of the transfer function (4) are ofcourse possible that may be employed that are effectively equivalent tothat mathematically expressed in equation (4) for purposes of enhanceaccuracy of the measurement of E as described herein. For example, theexpression just above may be replace by $\begin{matrix}{S_{A}^{\prime} = \frac{a - {Kb}}{a + {Kb}}} & \text{(4b)}\end{matrix}$Even though S′_(A) may have the benefit of better signal-to-noise ratiocompared to S_(A), the matters disclosed in this invention equally applywhether S_(A) or S′_(A), or the like, are used.

Also illustrated in FIG. 2 is a graphical depiction of S_(A) as afunction of time for the given electric field E, resultant sensor outputsignals a, b, and ratio K, where, P₀=1.0, K_(A)=0.9, K_(B)=0.81,K=1.111, α=0.90, Δφ_(o)=0.0, πE/E_(π)=0.01.

FIG. 3 illustrates a graphical depiction of a plot of the transferfunction (4) with for an example set of EFS sensing system parameters asjust recited. The most linear portion of the transfer function isgenerally indicated by the dashed rectangle.

The inverse transfer function is mathematically expressed as:$\begin{matrix}{E = {\frac{E_{\pi}}{\pi}\left\lbrack {{{- \Delta}\quad\varphi_{0}} + {{Arcsin}\left\lbrack {\left( \frac{2}{\alpha} \right)*\left( {\frac{a}{a + {Kb}} - 0.5} \right)} \right\rbrack}} \right.}} & (5)\end{matrix}$

Assuming the values of sensing system specific parameters α, K, Δφ_(o),and E_(π) are known, the value of the electric field may be calculatedas a function of resultant signals a and b using equation (5) by way ofsignal processor 800. A specific electric field sensing system, X, maybe characterize, in part, by empirically determining sensor systemparameter values for α and E_(π). For example, the values of α and E_(π)(or other values effectively equivalent to these two characterizingparameters) may be measured by calibrating the specific EFS sensingsystem under known electric fields before using the EFS sensing systemto measure unknown electric fields. The parameter “K” depends on all ofthe optical and electrical losses in the signal path for signals “a” and“b” and may be measured and/or controlled for a particular EFS sensingsystem as will be further described. The residual bias parameter of theEFS sensing system, Δφ_(o), depends on the actual bias, and, inaccordance with the present invention, may also be measured and/orcontrolled.

The electric field intended to be measured may be expressed in the formgiven by equation (3) given the assumption that the electric field isdominantly a simple sinusoidal alternating signal, as is typically thecase in electric power systems. Accordingly, the parameter K may bedetermined using equations (2) and (3). The parameter K is equal topeak-to-peak value (or some AC rms value) of sensor signal “a” dividedby the peak-to-peak (or similar AC rms) value of sensor signal “b”:$\begin{matrix}{K = {\frac{K_{A}}{K_{B}} = \frac{A_{AC}}{B_{AC}}}} & (6)\end{matrix}$where A_(AC) and B_(AC) are proportional to the absolute value of themagnitude (e.g., root-mean-square, peak-to-peak, or amplitude, or thelike) of the alternating components of sensor signals a and b. Forexample, for a value of residual bias Δφ_(o) close to zero, and for Emuch smaller than E_(π) (a very typical and practical case), one maywrite:a=A _(DC) +a _(AC)b=B _(DC) +b _(AC)ora≈A _(DC) +A _(AC) sin(ωt)  (7.a)b≈B _(DC) −B _(AC) sin(ωt)  (7.b)where,

-   -   A_(DC) and B_(DC) are the DC or average values of sensor signals        a and b, and    -   A_(AC) and B_(AC) are the amplitude of the alternating signal        components of sensor signals a and b.

Calculating K in this manner is essentially made possible by a keyproperty of the Pockels cell EFS: Namely, any light that is not guidedto channel A is guided to channel B, i.e., ideally A_(AC)=B_(AC) whichessentially is the basis of operation of the Pockels cell shown in FIG.1. In practice, the two sensor signal output components will experiencedifferent losses and gains in their optical paths and in theirelectronic signal paths such that A_(AC)=K×B_(AC).

Once K is calculated using equations (6) and (7) above, one maydynamically calculate Δφ_(o) using the mathematical expression ofequations (3) and (5): $\begin{matrix}{{E_{0}{\sin\left( {\omega\quad t} \right)}} = {\frac{E_{\pi}}{\pi}\left\lbrack {{{- \Delta}\quad\varphi_{0}} + {{Arcsin}\left\lbrack {\left( \frac{2}{\alpha} \right)*\left( {\frac{a}{a + {Kb}} - 0.5} \right)} \right\rbrack}} \right.}} & (8)\end{matrix}$

The average value of equation (8) over time is equal to zero, hence:$\begin{matrix}{{\Delta\quad\varphi_{0}} = {{Average}\left\{ {{Arc}\quad{\sin\left\lbrack {\left( \frac{2}{\alpha} \right)\left( {\frac{a}{a + {Kb}} - 0.5} \right)} \right\rbrack}} \right\}}} & (9)\end{matrix}$

Accordingly, for an alternating current signal, the parameters K andΔφ_(o) may be dynamically measured and adjusted. Note that theparameters K and Δφ_(o) may and probably will change over time asfunctions of various other parameters. For example, in an electric fieldsensor head, the parameter Δφ_(o) may change as a function of thetemperature, particularly that of the quarter-wave plate 26 of the inputoptical circuit illustrated in the embodiment shown in FIG. 1. Further,the parameter “K” may change due to drift in the gain of aphoto-detector in the detection unit over time. However, these changesare relatively slow.

In accordance with one embodiment of the present invention particularlyillustrated in FIG. 10, signal processor 800, specifically digitalsignal processor 920, is operative for periodically updating thecharacterizing parameters, K and Δφ_(o), i.e., periodically calculatingnew values for parameters K and Δφ_(o), and also tracking their valuesover time. For example, temperature may change at a rate of 20Kelvin/hour, and temperature dependence of Δφ_(o) may be ˜0.001radian/Kelvin, resulting in 0.02 radian/hour change in Δφ_(o).Parameters K and Δφ_(o) may be calculated every 10 seconds, for example,allowing for better than 0.02/360=0.000056 radian accuracy in knowingΔφ_(o) at any time. In summary, if any changes in K and/or Δφ_(o) aremeasured accurately, and therefore, K and Δφ_(o) are always well known,then they will not contribute to any significant error in use ofequation (5) or (8) for determining electric field E intended to bemeasured.

Furthermore, the ability to measure the parameters K and Δφ_(o)dynamically or frequently, allows for building highly accurate sensorsthat are relatively immune to temperature or light intensityfluctuations. For example, if over time (e.g., days), an amplifier thatamplifies the signal received on a photo-detector (where L_(B) ismeasured) has a gain change due to some thermal effect, i.e., K_(B) ischanged, calculating and adjusting K frequently will eliminate thepossibility of introducing an error in the electric field measurementwhen the inverse transfer function, equation (5) is used.

Temperature Sensor

In accordance with another aspect of the present invention particularlyillustrated in FIG. 11, tracking the parameter Δφ_(o) permits theelectric field sensor to be also used as a temperature sensor. The EFSsensing system may be characterized over temperature, in a test chamberfor example, so that the relationship between its bias, or residualbias, Δφ_(o), and temperature is well characterized. This isparticularly illustrated in FIG. 10 by way of a look-up table indicatedby block 987. Then the measurement of bias may be used to measure and/orestimate the ambient temperature at the sensor-head spatial location,thereby serving, in part, as a sensing head, and more particularly ahigh voltage insulator column temperature sensor.

FIG. 4 graphically depicts the relationship of the residual bias,Δφ_(o), as a function of temperature for an EFS. Knowledge oftemperature may be used for many applications, including compensatingfor temperature dependence of the sensitivity of the EFS itself. In asimplified form, the sensitivity of the EFS is mainly given by the valueE_(π) in equations (1), (2), (4), (5) and (8) above. E_(π) may bedetermined using equation (8) where all other sensor parameters andE_(o) are already known. Equation (5) may be rewritten as:$\begin{matrix}{Y = {{\frac{\pi\quad E}{E_{\pi}} + {\Delta\quad\varphi_{0}}} = {{Arc}\quad{\sin\left\lbrack {\left( \frac{2}{\alpha} \right)*\left( {\frac{a}{a + {Kb}} - 0.5} \right)} \right\rbrack}}}} & (10)\end{matrix}$

Using equations (3) and (10), the amplitude of the AC portion of Ydivided by E_(o), i.e., Y_(AC)/E_(o), will be inversely proportional toE_(π) and may be used as a measure of the sensitivity of the sensor. TheDC portion of Y is practically equal to Δφ_(o) (for E/E_(π) much smallerthan 1).

Accordingly, an electric field sensor may be characterized in thefollowing exemplary manner. Consider, for example, applying a voltage Vacross a pair of electrodes, which are separated at a constant distance,to create an electric field E between them in spatial proximity to anEFS sensor within a test chamber. For an applied sinusoidal voltagebetween the terminals, the voltage across the terminal may be expressedas:V=V _(o) sin(ωt+δ)=C E _(o) sin(ωt),  (11)where the parameters C and δ depend on the geometry and the propertiesof the materials in this environment, including the EFS sensor itself.

If we define R as:R≡Y _(AC) /V _(o),  (12)the value of the parameter R may be used as a measure of the sensitivityof the EFS.

In accordance with another aspect of the present invention, the EFS maybe characterized in terms of the parameter values “R” and Δφ_(o) as afunction of temperature. These parameters may be stored and utilize bythe signal processor 800, and more specifically by calculation block 950of digital signal processor 920 illustrated in FIG. 10, in order tocompensate for EFS sensing system output signals for temperaturevariations in the sensitivity of the EFS.

The signal processor 800, in accordance with one aspect of the presentinvention, is configured for re-calculating Δφ_(o) frequently (asexplained above) as particularly illustrated with reference to digitalsignal processor 920 illustrated in FIG. 10. Using the storedcharacterization information of the EFS sensing system, the temperatureof the EFS may be calculated (or looked up) from Δφ_(o); and again,using the stored characterization information, the sensitivityadjustment factor for the EFS at that temperature may be looked up (orcalculated).

Alternatively, instead of going through a two-step process, from Δφ_(o)to temperature, and from temperature to R, a single-step process mayemployed going directly from Δφ_(o) to R. The relationship betweenΔφ_(o) and R may be obtained through sensor sensing systemcharacterization, and may, for example, be stored as a look up table oras a predetermined function of Δφ_(o) calculated by way of signalprocessor 800.

FIG. 5 graphically depicts a plot of R as a function of temperature forthe same sensor of FIG. 4. FIG. 6 graphically depicts a plot of R as afunction Δφ_(o), combining the data from FIGS. 4 and 5. FIG. 7graphically depicts a plot of the inverse of R, namely 1/R, as afunction of Δφ_(o), that may be stored in the processing unit and beused for compensating the temperature dependent sensitivity variation ofthe EFS. In summary, using the above techniques, a “self temperaturecompensated” electric field sensor system may be obtained.

EFS Sensing System Condition Alarms

Another advantage of measuring and updating parameters K and Δφ_(o)frequently is that the measured values may be used for monitoring theEFS sensing system, and alarming against any unusual conditions; i.e.,allow for and improve “self monitoring” of the EFS sensing system. Thisaspect of the present invention is particularly illustrated in FIG. 11where digital signal processor 920 is configured to include parametermonitoring indicated by block 970 and a designated alarm indicator orfunction indicated by block 972 responsive to system monitoring block970. As indicated earlier, functions such as these and others may beaccomplished by way of hardware, software, firmware, and the like, thedetails of which are well within the skill of the artisan and notdescribed herein.

For one exemplary alarm function, if K becomes significantly larger than1 (e.g. K˜10), it could mean that the sensor “b” signal is becoming veryweak. Perhaps some optical connector in the path of that signal isdegrading, or a photo-detector in the path of that signal is failing.

Another exemplary alarm function is that if the residual bias is a valueout of the expected range for all possible temperature conditions, seeFIG. 4 for example, then the EFS sensor system may yield an alarmindicating that there is some problem in the EFS sensing system. Forexample, if at the installation of the EFS sensing system, the outputfibers that are supposed to be connected to channel A and channel Bphoto-detectors are reversed by mistake, the measured bias will bedifferent (typically reversed sign); e.g., for the EFS depicted in FIG.4, instead of Δφ_(o) being a value between 0.01 and 0.2 radians, it willbe a value between −0.01 and −0.2. In summary, many “self monitoring”features for the EFS sensing system may be implemented and/or enhancedusing these principles of the present invention.

The method taught above for measuring EFS sensing system parameters Kand Δφ_(o) uses the presence of an AC electric field at the location ofthe sensor to calculate K and Δφ_(o) accurately and frequently in orderto maintain accurate electric field measurement. If the AC electricfield is interrupted or distorted for a while, the accuracy of thesystem is typically still maintained since the values of K and Δφ_(o)don't typically change rapidly. Of course, some additional logic and/orprocessing may be required in order to make sure wrong values do not getcalculated.

For example, a logic rule may be as follows: if the values of A_(AC)and/or B_(AC) (see equation (7)) are less than certain minimum values,it should be interpreted as having no or low electric field at thelocation of the sensor and K should not be updated using these low and,perhaps, inaccurate values—thus avoiding an incorrect determination ofthe value of K and subsequent electric field measurements.

Zero Wake-Up Time Feature

In another scenario, when powering up a sensor system that useselectronics, for example, after any major service of the system, therewill be some delay time required (perhaps several seconds or minutes)for the electronics to boot up and warm up to equilibrium. This time isreferred to as “warm-up” time. Further, when any sensor system is usedfor measuring electric field, voltage, or current, of a powertransmission line, there may be a time delay between the energization ofthe power transmission line and the time when the output of the sensoris valid. This time delay is referred to as “wake-up” time.

It should be noted however that wake-up time should not be confused with“warm-up” time. Warm up time has to do with the “energization of theelectronics part of the sensor system”. The wake-up time is the time ittakes from “the presence of the parameter to be measured (e.g., HVrelated electric field) until it is correctly measured and presented.”The warm-up time may be several minutes without causing anyinconvenience in a permanent installation. However, the wake up time istypically expected to be very low, depending on the application.

The electric field sensors described herein may be used for measuringvoltage on HV transmission lines. The voltage may need to be known forvarious applications including protection and metering applications. Formetering applications the accuracy required is typically much morestringent, however a wake-up time of several seconds may be tolerated.For example, if the electric power to be measured needs to be accuratewith an uncertainty less than 0.2% for the monthly electricity bill(revenue metering application), >0.2% inaccuracy, or even total loss ofdata, over one minute will not affect the monthly bill significantly(one month is approximately 60×24×30=43200 minutes).

For a protection application, however, a very low wake-up time isrequired. Typically, a fault needs to be cleared in severalmilli-seconds after it appears on the power transmission line. Somefaults may happen exactly when the transmission line is energized. Inthese cases, the wake-up time, for protection applications, needs to bevery short, ideally zero. On the other hand, the accuracy requirementfor protection applications is not as stringent, for example, theInternational Standard—Instrument Transformers—Part2, IEC 60044-2,specifies a class 3P protection voltage sensor that may have anuncertainty of up to 3% in its measurement. The techniques explainedherein for continuous measurement and monitoring of K and Δφ_(o) implythat the wake-up time of the sensor is not zero.

The following exposition describes a technique used to achieve a “zerowake-up time” EFS sensing system of the type explained above so that onesensor may satisfy all the needs of both metering (accurate but slow)and protection (not so accurate but fast) applications.

The zero wake-up time function is particularly illustrated in thecombination block and flow diagram of FIG. 12 which depicts anotherfunctional arrangement for digital signal processor 920. The schemeemployed in accordance with the present invention may be described asfollows: the signal processor 800, as indicated in FIG. 1, may implementa start up routine 955 which uses the DC values of the resultant signalsa and b, A_(DC) and B_(DC), respectively, see equations (6) and (7), andthe characterization data, see FIGS. 4 to 7, to estimate bias (orresidual bias) first, then K may be calculated from bias and A_(DC) andB_(DC) in the absence of electric field (E=0) using equations (2) or (9)and (7): $\begin{matrix}{K = {\frac{A_{DC}}{B_{DC}} \times \frac{\left( {1 - {\alpha\quad{\sin\left( {\Delta\quad\varphi_{0}} \right)}}} \right.}{\left( {1 + {\alpha\quad\sin\quad\left( {\Delta\quad\varphi_{0}} \right)}} \right.}}} & (13)\end{matrix}$

The goal is to calculate the EFS sensing system parameters K and Δφ_(o)just accurately enough so that the errors in E associated withinaccuracies in K and Δφ_(o) when using the inverse transfer function ofequation (5) to measure E are sufficiently low for a protectionapplication.

So, for the protection application, a wake-up time of zero may beachieved in this manner. During the first few seconds after the ACvoltage becomes present on the HV transmission line, K and Δφ_(o) arerecalculated more precisely, as described previously, and higheraccuracy appropriate for metering is achieved. In other words, thewake-up time of the EFS sensing system for protection level accuracy iszero (where it matters), but the wake-up time for metering levelaccuracy is several seconds (where it does not cause any problems).

Using a specific example to show the “zero wake-up time” technique,assume an EFS sensing system having performance as depicted in FIGS. 4to 7, and that the sensor system needs to be used for both metering andprotection applications. For the metering application, it is expected tomeet IEC 60044-7 class 0.5 accuracy. Class 0.5 requires that theuncertainty or error in the amplitude of the AC signal measured by thesensor to be less than 0.5%. For the protection application, the sensoris expected to meet IEC 60044-7 standard class 6P. Class 6P requiresthat the uncertainty or error in the amplitude of the AC signal measuredby the sensor to be less than 6%.

FIG. 5 shows that if the EFS sensor is not compensated, the sensitivityof the electric field sensor, R, may vary by about ˜5% (from 1.70 to1.78). So, if R is assume to be 1.74 at all time, it (inaccurate R orE_(π)) will contribute between −2.5% and +2.5% error to the value of Ecalculated using the inverse transfer function of equation (5). Itleaves between −3.5% and +3.5% allowance for error contribution frominaccuracies in K and Δφ_(o).

FIG. 5, shows that the residual bias of the sensor will be between 0 and0.2 radians. If we assume the residual bias is 0.15 (Δφ_(oRef)=0.15), Kmay be calculated using equation (13) and the average values of sensorsignals a and b, A_(DC) and B_(DC), respectively: $\begin{matrix}{K_{initial} = {\frac{A_{DC}}{B_{DC}} \times \frac{\left( {1 - {\alpha\quad{\sin\left( {\Delta\quad\varphi_{0{Ref}}} \right)}}} \right.}{\left( {1 + {\alpha\quad{\sin\left( {\Delta\quad\varphi_{0{Ref}}} \right)}}} \right.}}} & (14)\end{matrix}$

FIG. 8 shows the error in the measurement of AC electric field performedby the EFS sensing system as a function of the true Δφ_(o), when a valueof Δφ_(oRef)=0.15 is used as explained in the previous paragraph.

FIG. 8 shows that the error in calculating electric field due toincorrect K and Δφ_(o), for all possible values of Δφ_(o) (between 0 and0.2 radian here) is less than 1%. For values E_(o)/E_(π) much less thanone (e.g., <0.03 is a common and practical case), the error in measuredelectric field E_(o) shown in FIG. 8 is proportional to the error in theslope of the transfer function assumed (error in slope˜[cos(Δφ_(oRef))/cos(Δφ_(o))−1], see equation (4)).

The “zero wake-up time” technique, exemplified above, uses thecharacteristic of the slope of the transfer function of the EFS sensingsystem to achieve moderate accuracy at start-up. For values of Δφ_(o)close to zero at all practical temperatures of interest, e.g., Δφ_(o)between −0.2 and 0.2 radian, which is a very easy and practical case,the transfer function (see FIG. 3, or equations (4) and (5)) is quitelinear with a relatively constant slope. Using incorrect bias and usingDC values of sensor signals a and b may contribute some error tocalculating K, but use of inaccurate K calculated in this manner doesnot result in unacceptably inaccurate AC electric field measurement (seeFIG. 8). Effectively, since the “wrong” K is based on a “wrong” Δφ_(o),they mostly cancel the effect of each other. While, this method may beused to achieve 6% accuracy upon HV start up for a protectionapplication, after the presence of AC signal on the HV line for a while,e.g., 10 seconds, accurate K and Δφ_(o) may be calculated as explainedin the earlier sections of this document to achieve better than 0.5%uncertainty required for metering in the example given above.

A simple procedure for choosing Δφ_(oRef) to be used in thezero-wake-up-time routine above may be given as follows:

If the residual bias, Δφ_(o), over the temperature range of interestchanges between two extremes, one may find the maximum and minimumvalues of cos(Δφ_(o)), cos_(max)(Δφ_(o)) and cos_(min)(Δφ_(o)),respectively, and useΔφ_(oRef)=±Arccos{[cos_(max)(Δφ_(o))+cos_(min)(Δφ_(o))]/2}  (15)

The choice of + or − sign depends on the typical value of Δφ_(o) overthe temperature range of interest for the particular sensor being used.If Δφ_(o) is typically positive, then positive is recommended to beused; if Δφ_(o) is typically negative, then negative sign is recommendedto be used; nevertheless, either will work.

Moreover, if the temperature of the sensor is known accurately, theΔφ_(o) versus temperature characterization table of the sensor, asgraphically depicted in FIG. 4, may be used to estimate Δφ_(oRef) veryaccurately so that it is very close to true value of Δφ_(o). In thiscase, a wake-up time of zero may be achieved while guaranteeing muchhigher accuracy (e.g., metering accuracy class 0.2% of IEC 60044-7). Ifthe temperature is not accurately known, but may be estimated with someerror bounds, e.g., ±10° C., the Δφ_(o) versus temperaturecharacterization table may be divided into a few regions, e.g., 6regions of 20° C. span to cover 120° C. temperature variation, and thenthe zero-wake-up-time routine given above may be used within each region(i.e., one Δφ_(oRef) calculated using equation (15) for each region) toguarantee more accurate measurement of electric field at HV energizationtime.

For example, for the EFS depicted in FIG. 4, the operating range may bedivided into 7 segments, each covering say, ˜20° C., while thecontribution of the zero-wake-up-time algorithm given above to error inelectric field measurement is less than 0.15%, and contribution oftemperature sensitivity of the sensor (see FIG. 5) is less than 0.35%,resulting in maintaining uncertainty less than 0.5% in measuringelectric field (or voltage) as soon as AC voltage is applied to the HVline (zero wake-up time).

The techniques given above may be used for other devices and sensorsthat have transfer functions of the form substantially given byequations (1) and (2). An example of that is the integrated opticsPockels cell described in the U.S. Pat. No. 5,029,273, issued to Jaeger,and further described in the following paper by Jaeger and Rahmatian:“Integrated Optics Pockels Cell High-Voltage Sensor,” IEEE Transactionson Power Delivery, Vol. 10, No. 1, January 1995, pp. 127-134, hereinincorporated by reference hereto.

In the explanations above, e.g., on self-temperature-compensatingelectric field sensor, the concepts of correction and compensation wereexplained using the amplitude of the AC signal as the parameter ofinterest. Obviously, the same techniques may be used to characterize andcorrect for other parameters such as the phase of the AC voltage to bemeasured, e.g., δ as particularly described in equation (11).

The following exposition sets forth an exemplary combination block andflow diagrams illustrating implementation of the aforesaid novelconcepts and techniques in accordance with the present invention. Asshould be appreciated, the description of the blocks and theirrespective functions may be implemented by a wide variety of analog andand digital components, including among others hardware, firm ware,software, and the like including use digital computers, digitalprocessor, i.e., microprocessors, and the like, all of which areintended to be within the true spirit and scope of the presentinvention.

Illustrated in FIG. 10 is an exemplary combination flow and blockdiagram for the digital signal processor 920 illustrating particularfunctions as mathematically described above. As before, digital signalprocessor 920 receives digital input signals D_(a) and D_(b). From thesesignals, values of K and Δφ_(o) may be calculated in accordance withequations 6 and 9, as designated by blocks 930 and 932 respectively. Inturn these values may be stored as indicated by blocks 940 and 942.Calculation block 950 is operative for calculation of the value Erepresentative of the electric field intended to be measured as afunction of the input signals D_(a) and D_(b), and the determined valuesof K and Δφ_(o).

Further illustrated in FIG. 10, is look-up table 987 which containssensing system specific characterizing information as that alreadydescribed for correcting and/or compensation the transfer function asdesired. More specifically, a table of temp, Δφ_(o), and 1R or R may beemployed by calculation block 950 in a manner as previously described.The aforesaid look-up table 987 has been intentionally omitted in FIGS.11 and 12 to enhance understanding of the invention, but which may beincorporated therewith as should be recognized by those killed in theart.

As illustrated in the exemplary combination flow and block diagram ofFIG. 10, values of K and Δφ_(o) may be periodically calculated orupdated in accordance with a preselected update time as generallyindicated by blocks 960 and 962, for generating a sampling signal asinputs to blocks 940 and 942.

As illustrated in the exemplary combination flow and block diagram ofFIG. 11, values of K and Δφ_(o) may be monitored as indicated by block970 including comparators for generating an alarm signal input to alarmindicating block 972. For example, the value of K may be monitoredand/or tracked to indicate for example the losses in theoptical-electrical circuitry are beyond acceptable range or non-existentindicating a fault, or that other catastrophic or performancecharacteristics as indicated by values of Δφ_(o) as aforesaid.

As is also indicated in FIG. 11, the value of Δφ_(o) may be signalprocessed to provide an output signal indicative of the temperature ofthe sensing-head as indicated by block 985 which also may be monitoredfor indicating a system fault in the high voltage environment, or otherfunctions.

It should be noted that the input and output optical circuits of thesensing head may be alternatively arranged to achieve the same intendedfunction. For example, quarter-wave plate 26 may alternatively place atthe output of the sensor 12 in order to obtain optical output lightwaves 47 and 49.

It should also be recognized that signal processor 800 including signalconditioning may be constructed from a combination of both digital ananalog circuits as desired, all of which are within the true spirit andscope of the present invention.

While the present invention has been particularly shown and describedwith reference to the accompanying figures, it will be understood,however, that other modifications thereto are of course possible, all ofwhich are intended to be within the true spirit and scope of the presentinvention. Various changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

Again, it should be noted that other transfer functions may be employedbeyond those mathematically expressed by equations 4 and 4a, all ofwhich are intended to be within the true spirit and scope of the presentinvention as already indicated.

More specifically, it should be recognized by those skilled in the artthat the electric field sensing head may take on the form of a widevariety sensor known or to become available that may be selected toachieve the same intended function as described herein, all of which areintended to be within the true spirit and scope of the presentinvention. Further, the present invention may employ a sensing headintended for sensing a different measure and, but which affects thetransit time of light wave components of differing polarization, and isalso intended to be within the true spirit and scope of the presentinvention.

Lastly, specific exemplary mathematical expressions have been set forthherein, however, other such expression are intended to be within thetrue spirit and scope of the present invention where parameter valuesrelated to the values of K and the residual bias are obtained, alongwith signals indicative of the pair of output light wave from theoptical sensing element are processed.

1. An electric field sensing system for measuring electric fieldcomprising: a light source for generating a primary light wave; firstand second photodetectors for providing first and second photodetectorelectrical output signals, respectively, in response to any lightimpinging thereon, respectively; an optical electric field sensing headhaving, at least a first optical sensing element that exhibits anelectro-optic effect where the transit time of light waves passingtherethrough of different polarizations are affected differently inresponse to an electric field thereat; a first optical circuitresponsive to said primary light wave for deriving first and secondlight wave components therefrom, and coupling said first and secondlight wave components to said optical sensing element, where said firstand second light wave components have different polarizations and have aphase separation, φ_(o), and a second optical circuit for deriving thirdand fourth light waves of different polarization from a light waveexiting from said first optical sensing element, and directing saidthird and fourth light waves to impinge upon said first and secondphotodetectors, respectively; and a signal processor for deriving aprocessor output signal, E, representative of the electric fieldintended to be measured as a sensing system specific, X, characterizedfunction, G_(x), mathematically described by:E=G _(x) {a, b, E _(π), φ_(o) , K, α} where, a and b represent resultantsignal processor input signals derived from the first and secondphotodetector electrical output signals, respectively; E_(π) is asensing system specific parameter being substantially the averageelectric field required to create a π radian phase shift between opticalpolarization components traveling from the input to the output of saidfirst optical sensing element in response to an electric field thereat,φ_(o) is the equivalent intrinsic phase shift between the first andsecond light wave components, α is a number between 0 and 1 thatcharacterizes the goodness of the specific electric field sensingsystem, X, and K is equal to the ratio K_(A)/K_(B), where K_(A) andK_(B) are characteristic loss factors of the specific electric fieldsensing system, X, associated with said first and second processor inputsignals, respectively.
 2. The electric field sensing system of claim 1wherein: said first optical circuit is operative such that said firstand second light wave components are substantially linearly polarizedlight wave components of orthogonal polarization and have a selectedphase difference therebetween; and said second optical circuit serves asa polarizing beam splitter.
 3. The sensing system of claim 1 where saidsensing system specific characterized function, G_(x), is of the form ofa substantially inverse sinusoidal function.
 4. An electric fieldsensing system for measuring electric field comprising: a light sourcefor generating a primary light wave; at least a first optical sensingelement that exhibits an electro-optic effect where the transit time oflight waves, therethrough, of different polarizations are affecteddifferently in response to art electric field thereat, said opticalsensing element having an input means for receiving light waves, and anoutput means from which light waves exit therefrom; a first opticalcircuit for deriving first and second light wave components of differentpolarizations from said primary light wave thereof, and coupling saidfirst and second light wave components to said input means of said firstsensing element; a second optical circuit for deriving third and fourthlight waves of different polarization from a light wave exiting fromsaid first sensing element output means; first and second photodetectorsfor providing first and second photodetector electrical output signalsin response to said third and fourth light waves impinging thereon,respectively; a signal processor responsive to first and secondprocessor input signals derived from said first and second photodetectorelectrical output signals for deriving therefrom a processor outputsignal, E, representative of the electric field intended to be measuredas a sensing system specific, X, characterized function, G_(x).
 5. Theelectric field sensing system of claim 4 wherein said sensing systemspecific characterized function. G_(x), is mathematically describe by:E=G _(x) {a, b, Eπ, φ _(o) , K, α} where, a and b represent resultantsignal processor input signals derived from the first and secondphotodetector electrical output signals, E_(π) is a sensing systemspecific parameter being substantially the average electric fieldrequired to create a π radian phase shift between optical polarizationcomponents traveling from the input to the output of said opticalsensing element in response to an electric field thereat, φ_(o) is theequivalent intrinsic phase shift between the first and second light wavecomponents, α is a number between 0 and 1 that characterizes thegoodness of the specific electric field sensing system, X, and K isequal to the ratio K_(A)/K_(B), where K_(A) and K_(B) are characteristicloss factors of the specific electric field sensing system, X,associated with said first and second processor input signals,respectively.
 6. The sensing system of claim 5 where said sensing systemspecific characterized function, G_(x), is of the form of asubstantially inverse sinusoidal function.
 7. The electric field sensingsystem of claim 4 wherein: said first optical circuit is operative suchthat said first and second light wave components are substantiallylinearly polarized light wave components of opposite orthogonalpolarization and have a phase difference close to 90 degrees; and saidsecond optical circuit serves as a polarizing beam splitter.
 8. A methodfor measuring electric field with an optical electric field sensingelement that exhibits an electro-optic effect where the transit time oflight waves passing therethrough of different polarizations are affecteddifferently in response to an electric field thereat, and includes aninput means for receiving light waves, and a light output means fromwhich light waves exit therefrom, the method comprising the steps of:generating a primary light wave; deriving, by way of a first opticalcircuit, first and second light wave components, of differentpolarizations from said primary light wave thereof, and coupling saidfirst and second light wave components to said input means of saidsensing element; deriving, by way of a second optical circuit, third andfourth light waves of different polarization from a light wave exitingfrom said sensing element output means; generating first and secondphotodetector electrical output signals in response to said third andfourth light waves impinging upon first and second photodetectors,respectively; deriving an output signal, E, representative of theelectric field intended to be measured by a specific electric fieldsensing system, X, as a sensing system specific characterized function,G_(x) of said first and second photodetector electrical output signals,where said sensing system specific characterized function, G_(x)characterizes, in part, at least system specific characteristics of saidfirst and second optical circuits, said first and second photodetectors,and said optical sensing element.
 9. The method of claim 8 furthercomprising the step of empirically determining a plurality of systemspecific parameters, and where said sensing system specificcharacterized function, G_(x), is a function of said plurality of systemspecific parameters.
 10. The method of claim 9 where said sensing systemspecific characterized function, G_(x) is mathematically described by:E=G _(x) {a, b, E _(π), φ_(o) , K, α} where, a and b represent resultantsignal processor input signals derived from the first and secondphotodetector electrical output signals. E_(π) is a sensing systemspecific parameter being substantially the average electric fieldrequired to create a π radian phase shift between optical polarizationcomponents traveling from the input to the output of said opticalsensing element in response to an electric field thereat, φ_(o) is theequivalent intrinsic phase shift between the first and second light wavecomponents. α is a number between 0 and 1 that characterizes thegoodness of the specific electric field sensing system, X, and K isequal to the ratio K_(A)/K_(B), where K_(A) and K_(B) are characteristicloss factors of the specific electric field sensing system, X,associated with said first and second processor input signals,respectively.
 11. The method of claim 10 where said sensing systemspecific characterized function, G_(x), is of the form of asubstantially inverse sinusoidal function.
 12. The electric fieldsensing system of claim 8 wherein; said first optical circuit isoperative such that said first and second light wave components aresubstantially linearly polarized light wave components of polarization,and having a phase difference close to 90 degrees; and said secondoptical circuit serves as a polarizing beam splitter.
 13. The method ofclaim 8 where said sensing system specific characterized function,G_(x), is of the form of a substantially inverse sinusoidal function.14. An optical sensing system for measuring a selected measure andcomprising: a light source for generating a primary light wave; firstand second photodetectors for providing first and second photodetectorelectrical output signals, respectively, in response to any lightimpinging thereon, respectively; an optical sensing head having, atleast a first optical sensing element that exhibits an optical effectwhere the transit time of light waves passing therethrough of differentpolarizations are affected differently in response to the measure andthereat; a first optical circuit responsive to said primary light wavefar deriving first and second light wave components therefrom, andcoupling said first and second light wave components to said opticalsensing element, where said first and second light wave components havedifferent polarizations and have a phase separation, φ_(o), and a secondoptical circuit for deriving third and fourth light waves of differentpolarization from a light wave exiting from said first optical sensingelement, and directing said third and fourth light waves to impinge uponsaid first and second photodetectors, respectively; and a signalprocessor for deriving a processor output signal, E, representative ofthe measure and intended to be measured as a sensing system specific, X,characterized function, G_(x), mathematically described by:E=G _(x) {a, b, E _(π), φ_(o) , K, α} where, a and b represent resultantsignal processor input signals derived from the first and secondphotodetector electrical output signals, respectively; E_(π) is asensing system specific parameter being substantially the average sensedmeasure and required to create a π radian phase shift between opticalpolarization components traveling from the input to the output of saidfirst optical sensing element in response to the measure and thereat.φ_(o) is the intrinsic phase shift between the first and second lightwave components, α is a number between 0 and 1 that characterizes thegoodness of the specific measure and sensing system, X, and K is equalto the ratio K_(A)/K_(B), where K_(A), and K_(B) are characteristic lossfactors of the specific measureand sensing system, X, associated withsaid first and second processor input signals, respectively.
 15. Thesensing system of claim 14 wherein said measureand is magnetic field.16. The sensing system of claim 14 wherein said first optical circuit isoperative such that said first and second light wave components aresubstantially linearly polarized light wave components of orthogonalpolarization.
 17. The sensing system of claim 14 wherein said firstoptical circuit is operative such that said first and second light wavecomponents are substantially circularly polarized light wave componentsof opposite polarization.
 18. The sensing system of claim 14 where saidsystem specific characteristics include temperature dependence andsignal loss characteristics.
 19. The sensing system of claim 14 wheresaid sensing system specific characterized function, G_(x), is of theform of a substantially inverse sinusoidal function.